Unsaturated carbonyl compounds such as unsaturated aldehydes and ketones are versatile and valuable compounds. Unsaturated monoaldehydes for example, are known to be used in the preparation of fragrances, while unsaturated ketones such as acetylcyclohexene, i.e. an unsaturated ketone wherein the carbonyl group is attached directly to a cycloalkenic group, are known to be used e.g. for the simultaneous preparation of adipic acid and acetic acid.
The preparation of unsaturated carbonyl compounds containing at least a cycloalkenic moiety in the molecule as described hereinbefore, is known e.g. from Tetrahedron Letters, Vol. 30, No. 11, pp 1357-1360, 1989. Said preparation relates to the Diels-Alder reaction of unsaturated carbonyl compounds and conjugated dienes in the presence of a Lewis acid catalyst such as catechol boron bromide. Although a wide range of such unsaturated carbonyl compounds is described, which compounds were generally prepared in a relatively high yield, the preparation described has a disadvantage in that the process uses already relatively expensive starting materials, and/or materials which are not always readily available, i.e. unsaturated carbonyl compounds. In U.S. Pat. No. 3,557,217 a process is described for the preparation of unsaturated carbonyl compounds, as described hereinbefore, which proceeds via the reaction of methyl vinyl ketone and a conjugated diene, the methyl vinyl ketone being a component of a by-product carbonyl stream from a process for the production of 1,3-butadiene, and in which stream acetone, methyl ethyl ketone, vinylcyclohexene, ethylbenzene, styrene and water are also present. Although this process uses a relatively cheap starting material, it is restricted to the production of unsaturated carbonyl compounds based on methyl vinyl ketone. Hence, it can be concluded that there is considerable need for improvement in the preparation of unsaturated carbonyl compounds as described hereinbefore.
The problem underlying the present invention is developing a process for the preparation of unsaturated carbonyl compounds, having at least a cycloalkenic moiety in the molecule, and which process does not have one of more of the disadvantages as mentioned hereinbefore.
As a result of extensive research and experimentation a process was developed for the preparation of unsaturated carbonyl compounds, of the type as described hereinbefore, by reaction of an allylic alcohol and a conjugated diene via the in situ formation of an unsaturated carbonyl compound. This process has the advantage in that it not only overcomes a separate preparation and isolation of the unsaturated carbonyl compound as employed in the process described in Tetrahedron Letters, Vol. 30, No. 11, pp 1357-1360, 1989, but moreover enables the preparation of a much wider range of the desired carbonyl compounds than would be possible according to the process as described in U.S. Pat. No. 3,557,217.